1 corrosion and protection of steel reinforced concrete
CORROSION AND PROTECTION OF STEEL REINFORCED CONCRETE PROVIDED BY: EMAD BEHDAD LECTURER: PROF.SHAMS
OUTLINE• INTRODUCTION• CORROSION PROCESS• TYPES OF CORROSION• CAUSES OF CORROSION• PROTECTION METHODS• CONCLUSION
ASTM terminology (G 15) defines corrosion as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.” For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of the original steel and none of the good mechanical properties. Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.
Electrochemical process of steel corrosion in concrete
• Ca, Na, K hydroxides in hydrated cement raise the pH to ~13.5 A dense protective ferric oxide (Fe2O3) passive film forms around the reinforcement Passive film develops on the bar surface• This passive film stops iron dissolution, and is stable at pH >10 pH >13
Yes if: (a) Concrete is always dry, then there is no H2O to form rust. Also aggressive agents cannot easily diffuse into dry concrete. (b) Concrete is always wet, then there is no oxygen to form rust. (c) Cathodic protection is used to convert all the reinforcement into a cathode using a battery. This is not easy to implement because anodic mesh is expensive, and this technology is not easy to install and maintain
(d) A polymeric coating is applied to the concretemember to keep out aggressive agents. These areexpensive and not easy to apply and maintain.(e) A polymeric coating is applied to thereinforcing bars to protect them from moisture andaggressive agents. This is expensive and there issome debate as to its long- term effectiveness.(f) Stainless steel or cladded stainless steel is usedin lieu of conventional black bars. This is muchmore expensive than black bars.
Can we avoid corrosion? No, not entirely: Concrete is not usually under water or continuously dry. Aggressive agents such as carbon dioxide, de-icing agents and/or sea water can diffuse into the best of moist concrete, and corrosion will eventually result.
COMMON CORROSION TYPES1) Crevice CorrosionCrevice corrosion is a localized form of corrosion usuallyassociated with a stagnant solution on the micro-environmentallevel. Such stagnant microenvironments tend to occur in crevices(shielded areas). Oxygen in the liquid which is deep in the creviceis consumed by reaction with the metal. Oxygen content of liquidat the mouth of the crevice which is exposed to the air is greater,so a local cell develops in which the anode, or area being attacked,is the surface in contact with the oxygen-depleted liquid.
Crevice Corrosion of Rebar Has Some Similarities with Filliform CorrosionThe head of the advancing filament becomes anodic, with a low pH and a lack of oxygen, as compared with the cathodic area immediately behind the head where oxygen is available through the semipermeable film. Corrosion proceeds as the cathode follows behind the anodic head (from Corrosion Basics NACE).
2) PittingTheories of passivity fall into two general categories, one based on adsorption andthe other on presence of a thin oxide film. Pitting in the former case arises asdetrimental or activator species, such as Cl-, compete with O2 or OH- at specificsurface sites. By the oxide film theory, detrimental species become incorporatedinto the passive film, leading to its local dissolution or to development ofconductive paths. Once initiated, pits propagate auto-catalytically according to thegeneralized reaction, M+n + nH2O + nCl- → M(OH)n + nHCl, resulting in acidificationof the active region and corrosion at an accelerated rate (M+n and M are the ionicand metallic forms of the corroding metal).
ChloridesAirborne, marine, industrial, groundwater, cast-inCl– can penetrate through the passive filmAt Cl- > “threshold”, passive film breaks down, corrosion initiatesCl- “threshold” value is typically 0.05% by wt of concrete (0.02% prestressed concrete)Pitting corrosionChlorides are main cause of reinforcement corrosion
EFFECT OF CARBONATION It can cause soft surface, dusting and color change It reduces quality concrete It reduces the concrete ability to protect reinforcement from corrosion (in an exposed environment) It will result in additional shrinkage in carbonated region.
DETECTING CARBONATION Depth of carbonation can be detected using an indicator. A chemical such as Phenolphthalein sprayed on to freshly broken concrete. Areas remaining alkaline will turn in a bright purply-pink color. Carbonated areas of concrete will remain unchanged in color.
Reinforcing steel corrosion Migration of chlorides, H20 Corrosion of the steel and O2 into the concrete, no reinforcement and corrosion and no damage to cracking and/or spalling concrete of concreteDegree of Corrosion Initiation Propagation (corrosion) Critical chloride threshold I Time
EPOXY‐COATED BARS Anode Cathode Reduces anode area Reduces cathodic area Increases threshold* REDUCED CORROSION Electrical Connection Ionic path• Electrical path between anode and cathode Makes ionic pathway longer
thermally sprayed coatings of Zn and Al, combat corrosion For atmospheric, buried, and marine environment corrosion protection, Zn (TSZ), Al (TSA), and their alloys have proven that they provide long term corrosion protection and outperform most all other methods. Anodic (TSZ/TSA) metal coatings applied to steel cathodes (more noble than Zn or Al), are referred to as cathodic or sacrificial protection coating systems. These thermal spray coatings provide corrosion protection by excluding the environment (or electrolyte) and acting as a barrier coating (like paints, polymers, and epoxies), but unlike typical barrier coatings they also provide sacrificial anodic protection.
Zinc and zinc alloys are also sprayed directly onto concrete to protect the steel rebar withinArc spraying of zinc on a concrete bridge pierin the Florida Keys. In this case the zinc acts assacrificial anode, although it is more frequentlyused in impressed-current systems. Threeimpressed-current zinc systems have alreadybeen installed by the Ministry ofTransportation of Ontario in TorontoSacrificial cathodic protection of steel inconcrete by thermal zinc spraying
FLY ASH using a Fly Ash concrete with very low permeability, which will delay the arrival of carbonation and chlorides at the level of the steel reinforcement. Fly Ash is a finely divided silica rich powder that, in itself, gives no benefit when added to a concrete mixture, unless it can react with the calcium hydroxide formed in the first few days of hydration. Together they form a calcium silica hydrate (CSH) compound that over time effectively reduces concrete diffusivity to oxygen, carbon dioxide, water and chloride ions. By reducing ion diffusion, the electrical resistance of the concrete also increases
CATHODIC PROTECTIONImpressed current (active)Sacrificial anode (passive)
TITANIUM ANODE MESHA. TYPICALLY ATTACHED TO THE CONCRETE SURFACE ANDTHEN ENCAPSULATED IN CEMENTITIOUS MATERIALS.B- EASILY CONFORMS TO THE STRUCTURE GEOMETRY.C- MOST USED IMPRESSED CURRENT ANODE FOR CONCRETE.
Mixed Metal Oxide activated Titanium Anodes in the form of a ribbon mesh can be installed in close proximity and parallel to the reinforcement bars (rebar). MMO Ribbon Mesh
1. Simple to Install.2. No Power Supply Needed.3. No Wiring or Conduit.4. No Long-Term Monitoring or Maintenance